1. Field of the Invention
The present invention relates generally to an air/fuel ratio control system for an internal combustion engine. More specifically, the invention relates to an asynchronous fuel delivery control in an air/fuel ratio control for improving an engine driving characteristics.
2. Description of the Background Art
As is well known, asynchronous fuel delivery is performed in response to a demand of acceleration enrichment. Especially, such asynchronous fuel delivery is important for providing better acceleration characteristics in a single-point injection type fuel injection system for an internal combustion engine. Generally, acceleration enrichment demand is detected by means of an idle switch position or throttle valve angular position. In case that the idle switch position is taken as a parameter for detecting acceleration, switching of idle switch from ON position to OFF position is detected to detect the acceleration enrichment demand. On the other hand, when the throttle angular position is taken as parameter for detecting the acceleration enrichment demand, variation rate of throttle angular position is compared with a predetermined threshold in order to detect the engine driving condition requiring enrichment for acceleration. Asynchronous fuel injection is performed when acceleration enrichment demand is detected based on the selected parameter as set forth above. Fuel injection amount to be injected in asynchronous injection is derived on the basis of an engine coolant temperature and magnitude of acceleration demand, such as the throttle angle variation rate.
Fuel injection amount control in asynchronous injection has been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 59-200034.
When asynchronous fuel injection for acceleration enrichment is controlled on the basis of the idle switch position, there is no way for distinguishing moderate acceleration and abrupt or swift acceleration. Therefore, in either condition, same acceleration enrichment wil be performed to make an air/fuel mixture excessively rich in moderate acceleration or to make air/fuel ratio excessively lean in abrupt acceleration.
On the other hand, by taking the throttle valve angular position as parameter for detecting the acceleration enrichment demand, moderate acceleration and abrupt acceleration can be distinguished. However, in such case, an enrichment demand for moderate acceleration will be ignored. Namely, as long as the throttle angle variation rate is held smaller than the threshold, asynchronous fuel injection is not performed. In such throttle angle variation range, variation rate of the throttle angular position is not reflected to engine acceleration characteristics. This tends to degrade engine acceleration in certain range of engine acceleration.